1. Field of the Invention
This invention relates to azo colorants such as azo dyes and their use in contact with metal surfaces. In one particular application, this invention finds use in colored foodstuffs and beverages.
2. The Prior Art
Azo dyes are well known materials. A wide variety of "azo" compounds are known. In a general sense, an azo compound is a compound having a nitrogen-nitrogen double bond (--N=N--) in its structure. Among azo compounds known in the art may be found so-called "monoazo" compounds -- that is, compounds containing one --N=N-- bond, such as tartrazine, ##SPC1##
And "polyazo" compounds -- that is, compounds which within their unit chemical structure contain a plurality of azo bonds, such as CI Direct Brown 138, ##SPC2##
Many azo compounds have the property of being colored. This property has led to the wide adoption of azo compounds as dyes and colorants.
Conventional mono- and polyazo compounds have an undesirable property which can interfere with their use as colorants, however. They are corrosive to a range of common metals -- most importantly, tin, zinc, aluminum and steel, but also alloys thereof. Thus, when a material containing an azo compound, such as a colorant, is contacted with a surface made of or coated with these metals, corrosion occurs. This leads to contamination of the azo compound-containing material with corrosion products, such as metal flakes and ions, and to destruction of the metal surface. This rate of corrosion is a direct function of the concentration of azo compound, a fact which leads to less-than-desired amounts of azo compounds being added in an attempt to reduce corrosion.
This corrosion problem is most apparent in water-containing materials, especially foods and beverages. Beverages, such as colas and fruit-flavored soft drinks, generally are colored by azo colorants. This can lead to corrosion of food and beverage handling apparatus, apparatus which should be maintained scrupulously clean and noncorroded; and to unhealthful contamination of the foods and beverages. Moreover, this problem becomes even more acute in the case of canned foods and beverages. Can and metal container manufacturers are aware of this problem and often refuse to guarantee the soundness of their containers for extended periods of azo compound usage. This leads to short shelf lives for many common canned products, and to the use of lower amounts of colorants. As an example of this phenomenon, a comparison of canned and bottled orange soda generally reveals that the canned orange soda is much paler in color and less desirable.
Substitution of non-azo colors often is not a suitable solution to the problem, as many colored non-azo compounds are toxic, do not have desired properties, or are not approved by regulatory agencies for use in foods. A partial solution has been to coat the metal surface with polymeric organic resins to physically prevent the azo colorants from contacting the metal surfaces. This approach is expensive unless thin coatings are used. These thin coatings generally have imperfections and pin holes which expose the metal surface to the azo colorant and corrosion.